TRANSLOCATION IN POLYTRICHUM COMMUNE (BRYOPHYTA) I. CONDUCTION AND ALLOCATION OF PHOTOASSIMILATES

Leafy stems and connecting underground rhizomes of Polytrichum commune Hedw. contain leptome tissues similar in structure to phloem. Isolated stems in clonal groupings were pulse labelled with ¹⁴CO₂. Labelled sugar, mostly sucrose, glucose, and fructose, appeared in the pulse labelled stems 30 min after treatment. A small amount (3.3%) of labelled sugar was transported to neighboring stems. Silver grain deposition in microautoradiographs of interconnecting rhizomes occurred predominantly over leptome tissues. Increased amounts of translocated radioactivity appeared in starch and cell wall polysaccharide pools one week and six weeks after treatment. These results 1] indicate that transport of photoassimilate occurs through the leptome of perennating rhizomes, 2] demonstrate that translocated carbon is subsequently utilized or stored, and 3] raise important questions about the significance of long distance transport in the life strategy of this complex clonal moss.     

Living on a high sugar diet: the fate of sucrose ingested by a phloem-feeding insect,the pea aphid Acyrthosiphon pisum

The natural diet of aphids, plant phloem sap, generally contains high concentrations of sucrose. When pea aphids (Acyrthosiphon pisum) were fed on chemically defined diets containing sucrose radiolabelled in the glucose or fructose moiety, 2 to 12-fold and 87 to 110-fold more radioactivity was recovered from the tissues and honeydew, respectively, of aphids that ingested [U-(14)C-glucose]-sucrose than from those ingesting [U-(14)C-fructose]-sucrose. The total radioactivity recovered was 70% of the ingested [U-(14)C-glucose]-sucrose and <5% of ingested [U-(14)C-fructose]-sucrose. The dominant honeydew sugars produced by aphids feeding on 0.75 M sucrose diets were oligosaccharides comprising glucose. In vitro the guts of pea aphids had high sucrase activity, 1-5 U mg(-1) protein, generating equimolar glucose and fructose except at high sucrose concentrations where glucose production was inhibited (K(si)=0.1 M). These data suggest that the fructose moiety of ingested sucrose is assimilated very efficiently and may be preferentially respired by the aphid, and that the glucose moiety of sucrose is incorporated into oligosaccharides by the transglucosidase activity of the gut sucrase at high sucrose concentrations. These differences in the fate of sucrose-derived glucose and fructose are important elements in both the carbon nutrition and osmoregulation of aphids.     

The effect of season of growth on the chemical composition of cambial saps of Eucalyptus regnans trees

Summary Bark was stripped, at monthly intervals, from the stems of ten previously-unsampled trees of Eucalyptus regnans F. Muell. The exposed surfaces of inner phloem and outer xylem yielded phloem and cambial saps which were rapidly frozen. After freeze drying to determine the contents of water and dry-matter, the samples were extracted with 80% ethanol. The main components in this extract are low molecular weight carbohydrates and salts of inorganic acids. The carbohydrates comprise stachyose, raffinose, sucrose, galactinol, glucose, fructose, myo-inositol and galactose; sucrose is invariably the major component. The amounts of all components varied widely during the sampling period. Multiple regression analyses showed that season of growth has a significant effect on sucrose, glucose, fructose, total sugars and soluble dry-matter, maxima being recorded near the beginning of autumn and spring, and minima near the beginning of winter and summer; that oligosaccharide and myoinositol contents are significantly related to atmospheric temperature; and that rainfall has a significant effect on the hexose and total sugar contents, saps from the xylem surfaces being more affected than those from the phloem surfaces. The translocated photosynthates in E. regnans appear to be oligosaccharides of the raffinose family and sucrose. Significant negative correlations between oligosaccharides and both sucrose and myoinositol, and significant positive correlations between sucrose and both glucose and fructose, are consistent with enzymic hydrolysis and resynthesis of most di- and oligosaccharides. The biosynthetic demands of developing secondary tissues and/or the fluctuations in composition of sieve-tube assimilates appear to control the composition of the sugars in the saps. Oligosaccharides and sucrose may function as soluble reserve substances as well as translocated photosynthates. It is possible that myoinositolis a key component in the interconversion processes of the sugars; experiments with radioactive sugars tend to lend support to this contention, especially during winter conditions.     

Pathway of Phloem unloading of sucrose in corn roots

The pathway of phloem unloading and the metabolism of translocated sucrose were determined in corn (Zea mays) seedling roots. Several lines of evidence show that exogenous sucrose, unlike translocated sucrose, is hydrolyzed in the apoplast prior to uptake into the root cortical cells. These include (a) presence of cell wall invertase activity which represents 20% of the total tissue activity; (b) similarity in uptake and metabolism of [¹⁴C]sucrose and [¹⁴C]hexoses; and (c) randomization of ¹⁴C within the hexose moieties of intracellular sucrose following accumulation of [¹⁴C] (fructosyl)sucrose. Conversely, translocated sucrose does not undergo apoplastic hydrolysis during unloading. Asymmetrically labeled sucrose ([¹⁴C](fructose)sucrose), translocated from the germinating kernels to the root, remained intact indicating a symplastic pathway for unloading. In addition, isolated root protoplasts and vacuoles were used to demonstrate that soluble invertase activity (V_max = 29 micromoles per milligram protein per hour, K_m = 4 millimolar) was located mainly in the vacuole, suggesting that translocated sucrose entered via the symplasm and was hydrolyzed at the vacuole prior to metabolism.     

Interconversions of 14C-labelled Sugars in Apple Tree Tissues12

Tissue pieces excised from orchard-grown apple trees duringa growing season exhibited different and changing capabilitiesof transferring ¹⁴C-label from sucrose, fructose and sorbitolto other soluble carbohydrates. All tissues incorporated fructose¹⁴C into sucrose but only leaves incorporated significant amountsof label from sucrose into sorbitol. As seeds developed andmatured, their ability to incorporate ¹⁴C from sorbitol andfrom fructose into sucrose increased. Sorbitol and sucrose arethe major translocated photosynthetic products of apple leavesbut whereas sorbitol appears to be an end-product of synthesis,sucrose may be considered as a substrate involved more directlyin carbohydrate utilization. Key words: Malus domestica, Apple, Carbohydrate interconversions     

Soluble carbohydrates of Pinus sylvestris L. sapwood and heartwood

Summary The amounts of glucose, fructose, sucrose, arabinose/galactose, raffinose/stachyose and starch were investigated in the outer sapwood, innermost sapwood, transition zone and heartwood of four stems of Pinus sylvestris L. The samples were taken in October and the determination of the compounds was done enzymatically. It was not possible to distinguish arabinose from galactose and raffinose from stachyose. The amounts of glucose, fructose and sucrose were greatest in the outer sapwood and decreased gradually towards the innermost sapwood and the heartwood. In the outermost heartwood glucose, fructose and sucrose were only present in trace amounts. Raffinose/stachyose showed highest concentrations in the outer sapwood and decreased towards the heartwood. In contrast, the concentrations of arabinose/galactose increased towards the heartwood and the greatest amount was found in the inner heartwood. When identified by thin-layer chromatography (TLC), arabinose was found to be present in greater amounts than galactose. The amount of starch decreased markedly towards heartwood. However, the amounts of sugars in all the studied stems was very variable. The changes in the amounts of carbohydrates in the different zones of the stems and the possible relationships of these phenomena with heartwood formation are discussed.     

Crystal structure of the Glu328Gln mutant of Neisseria polysaccharea amylosucrase in complex with sucrose and maltoheptaose

Several enzymes acting on sucrose are found in glycoside hydrolase family 13 (the α–amylase family). They all transfer a glucosyl moiety from sucrose to an acceptor, but the products can be very different. The structure of a variant of one of these, the Glu328Gln mutant of Neisseria polysaccharea amylosucrase, has been determined in a ternary complex with sucrose and an oligosaccharide to 2.16 Å resolution using x-ray crystallography. Sucrose selectively binds in the active site and the oligosaccharide only binds at surface sites. When this structure is compared to structures of other enzymes acting on sucrose from glycoside hydrolase family 13, it is found that the active site residues are very similar around the glucose part of sucrose while much variation is seen around the fructose moiety.     

High Performance Liquid Chromatography Analysis of Carbohydrates of Cotton-Phloem Sap and of Honeydew Produced by Bemisia tabaci Feeding on Cotton

Phloem sap from cotton (Gossypium hirsutum L.) was collected from young and mature leaves by the aphid-stylet technique. Exudate was analyzed for carbohydrates by HPLC using sensitive pulsed amperometric detection. The predominant carbohydrate present (>90%) was identified as sucrose. A second, unidentified compound that was not one of the more commonly translocated sugars was detected in mature leaves. Carbohydrates in honeydew produced by the sweet-potato whitefly (Bemisia tabaci [Genn.]) feeding on cotton were sucrose, glucose, fructose, trehalulose, and a series of oligosaccharides.     

Inhibition of sucrose phosphatase by sucrose

1. Partially purified sucrose phosphatase from immature stem tissue of sugarcane is inhibited by sucrose. The enzyme was also inhibited by maltose, melezitose and 6-kestose but not by eight other sugars, including glucose and fructose. 2. The relative effectiveness of sucrose, maltose and melezitose as inhibitors is different for sucrose phosphatase from different plants. 3. The inhibition of the sugar-cane enzyme by sucrose was shown to be partially competitive. The K(i) for sucrose is about 10mm. 4. Melezitose is also a partially competitive inhibitor of the enzyme but the inhibition by maltose is probably mixed. 5. The possibility that sucrose controls both the rate of accumulation of sucrose in stems of sugar-cane and sucrose synthesis in leaves by inhibiting sucrose phosphatase is discussed.     

Amylosucrase from Neisseria polysaccharea: novel catalytic properties

Amylosucrase is a glucosyltransferase that synthesises an insoluble alpha-glucan from sucrose. The catalytic properties of the highly purified amylosucrase from Neisseria polysaccharea were characterised. Contrary to previously published results, it was demonstrated that in the presence of sucrose alone, several reactions are catalysed, in addition to polymer synthesis: sucrose hydrolysis, maltose and maltotriose synthesis by successive transfers of the glucosyl moiety of sucrose onto the released glucose, and finally turanose and trehalulose synthesis - these two sucrose isomers being obtained by glucosyl transfer onto fructose. The effect of initial sucrose concentration on initial activity demonstrated a non-Michaelian profile never previously described.     

Plasmid-mediated uptake and metabolism of sucrose by Escherichia coli K-12.

The conjugative plasmid pUR400 determines tetracycline resistance and enables cells of Escherichia coli K-12 to utilize sucrose as the sole carbon source. Three types of mutants affecting sucrose metabolism were derived from pUR400. One type lacked a specific transport system (srcA); another lacked sucrose-6-phosphate hydrolase (scrB); and the third, a regulatory mutant, expressed both of these functions constitutively (scrR). In a strain harboring pUR400, both transport and sucrose-6-phosphate hydrolase were inducible by fructose, sucrose, and raffinose; if a scrB mutant was used, fructose was the only inducer. These data suggested that fructose or a derivative acted as an endogenous inducer. Sucrose transport and sucrose-6-phosphate hydrolase were subject to catabolite repression; these two functions were not expressed in an E. coli host (of pUR400) deficient in the adenosine 3-,5'-phosphate receptor protein. Sucrose uptake (apparent Km = 10 microM) was dependent on the scrA gene product and on the phosphoenolpyruvate-dependent sugar:phosphotransferase system (PTS) of the host. The product of sucrose uptake (via group translocation) was identified as sucrose-6-phosphate, phosphorylated at C6 of the glucose moiety. Intracellular sucrose-6-phosphate hydrolase catalyzed the hydrolysis of sucrose-6-phosphate (Km = 0.17 mM), sucrose (Km = 60 mM), and raffinose (Km = 150 mM). The active enzyme was shown to be a dimer of Mr 110,000.     

Production of microbial levan from sucrose,sugarcane juice and beet molasses

Summary Bacillus polymyxa (NRRL-18475) produced a levan-type fructan (B, 26 fructofuranoside) when grown on sucrose, sugarcane juice, and sugarbeet molasses. The organism converted about 46% of the fructose moiety of sucrose to levan when grown on sucrose medium, however, the yields of levan from sugarcane juice and beet molasses were much less than sucrose solution. Such sugarcane juice and beet molasses can be made a good substrate for levan production by various modifications. Adding peptone to sugarcane juice or passing beet molasses through a column of gel filtration media improved levan yield to a level almost comparable to that obtained from sucrose.     

Comparative Distribution and Metabolism of Xylem-Borne Amino Compounds and Sucrose in Shoots of Populus deltoides

The transport and metabolism of xylem-borne amino compounds and sucrose were investigated in rapidly growing shoots of cottonwood (Populus deltoides Bartr. ex Marsh.). ¹⁴C-labeled glutamine, threonine, alanine, glutamic acid, aspartic acid, and sucrose were applied to the base of severed stems for transport in xylem. Distribution and metabolism of the compounds were followed with autoradiography, microautoradiography, and radioassay. Three utilization patterns were observed: (a) little alanine and sucrose was transported to the laminae of either mature leaves or developing leaves. These compounds were taken up from xylem free-space and utilized in adjacent tissue; (b) threonine also did not move into mature leaves but was translocated to developing leaves or utilized in the stem; (c) glutamic acid and aspartic acid were transported directly into the laminae of mature leaves via the xylem. Relatively less ¹⁴C was retained in stems compared to the other compounds.

Metabolism of the test compounds also differed considerably. ¹⁴C from amino acids moved primarily into organic acids and protein. The ¹⁴C from sucrose was widely distributed among the chemical fractions, with a high percentage found in structural carbohydrates. Clearly, cottonwood stems contain efficient uptake and transfer systems that differentiate among various compounds moving from root to shoot in xylem.

     

Characteristics of the phloem path: analysis and distribution of carbohydrates in the petiole of Cyclamen

Compartmentation fluxes of carbohydrates along the phloem path were analysed in the petiole of Cyclamen persicum (L.) Mill. Sucrose represented the dominant fraction (58-75% of soluble carbohydrates in the vascular symplast). Planteose (12-22%), glucose (3-8%) and fructose (3-13%) occurred in lower amounts (data from liquid chromatography, percentages of the total peak area). Starch was not detectable. Upon feeding leaves with ¹⁴CO2, 98% and 90% of radiolabel was recovered as sucrose in the vascular symplast after 3 h and 24 h, respectively. Thus, sucrose appeared to be the exclusive transport sugar in Cyclamen. Experiments with asymmetrically labelled sucrose revealed that there was no metabolism of translocated sucrose. Analysis of six consecutive petiole segments (each 2 cm in length) showed a homogeneous longitudinal distribution of these sugars differed markedly. On average, the sucrose concentration amounted to 4.7 and 0.4 mg g^-1 FM in the vascular apoplast and petiole parenchyma, respectively. Sucrose was unloaded with out hydrolysis and stored in the periphery of the phloem path. Planteose was identified as another storage saccharide. Sucrose synthesis by sucrose phosphate synthase occurred when isolated vascular bundles were incubated with [¹⁴C]glucose or [¹⁴C]fructose. These data suggest that the phloem path is characterized by both source and sink like activity.     

Glycogen synthesis by amylosucrase from Neisseria perflava.

Amylosucrase (sucrose:1,4-alpha-D-glucan 4-alpha-glucosyltransferase; EC 2.4.1.4) which mediates the transfer of the glucosyl moiety of sucrose to a growing alpha-1,4-glucan chain is a constitutive enzyme of Neisseria perflava. The products of enzymic action are insoluble glycogenlike polysaccharides and fructose, the latter being a competitive inhibitor of the enzyme (Ki=20 mM). The enzyme is extremely stable and appears to bind very tightly to its polymerized product. Properties of product-bound enzyme reflect those of the native complex.     

Sugar Selectivity and Other Characteristics of Phloem Loading in Beta vulgaris L

The rate of phloem loading, its selectivity, and the disposition of labeled carbon were studied following application of (14)C-labeled sugars to the free space of source leaves of sugar beet (Beta vulgaris L.). Buffered 10 mm solutions of (14)C-labeled sucrose, fructose, stachyose, mannitol, 3-0-methyl glucose or l-glucose were applied to the abraded epidermis of source leaves held in the dark. Distribution of the labeled carbon from sugar taken up from the free space was studied by micro-densitometry of autoradiographs. Uptake of labeled sugar from the free space, partition between mesophyll and minor veins, metabolic conversions, export and respiration were followed during the 3-hr time course studies. Rates of sugar uptake into the minor veins, flux rates through the sieve element-companion cell complex membrane and concentration ratios between free space and the interior of the minor vein phloem cells were compared for the six sugars studied for evidence of active uptake. The composition of the free space solution in leaves photosynthesizing in (14)CO(2) was studied by vacuum infiltration of the source leaf air spaces and removal of the solution by centrifugation. Labeled compounds in this solution were compared to those in an aqueous ethanol extract of the same leaf pieces.The results in sugar beet source leaves support the concept of direct, active uptake of sucrose from free space into minor veins. This is not the case for fructose, 3-0-methyl glucose, mannitol, or stachyose. The latter two sugars, which are translocated in some plants, are not loaded into the minor veins at a rate sufficient to make them a significant component of the material translocated. The rate of phloem loading is controlled in part by mesophyll metabolism, especially as it affects the availability of sucrose to the free space. Both the rate and selectivity of export are controlled by uptake from the free space into the sieve element-companion cell complex of the minor veins.     

Fructose and glucose mediates enterotoxin production and anaerobic metabolism of Bacillus cereus ATCC14579T

Aims:  To determine the effects of carbohydrates on Bacillus cereus ATCC14579^T anaerobic metabolism and enterotoxin production in amino acids rich medium.
Methods and Results:  Bacillus cereus anaerobic growth on different carbohydrates (glucose, fructose, sucrose or glucose–fructose mixture) was examined in synthetic mMOD medium under continuous cultures (μ = 0·2 h⁻¹). Fermentation end-products, flux partitioning at each key branch points of the mixed acid pathway and consumption or production of amino acids were determined. On both fructose and sucrose, ATP production was favoured via acetate production from acetyl-CoA. In addition, amino acids present in the growth medium showed significant variations with high consumption of serine and net production of glutamate and alanine on some or all sugars. Enterotoxins Hbl and Nhe production was high during growth on fructose (or mixtures involving a fructose moiety).
Conclusions:  Fructose was identified as a key sugar influencing anaerobic metabolism and toxin production of B. cereus .
Significance and Impact of the Study:  The physiological differences associated with the fermentation of the various carbohydrates clearly modify toxinogenesis indicating that the risk of foodborne pathogens is to some extent dependent upon the prevailing nutritional environment.     

Differential Rates of Uptake of Glucose, Fructose and Sucrose by Pea Stem Protoplasts

Protoplasts from growing regions of etiolated pea stems takeup glucose more rapidly than fructose when supplied for briefperiods at low concentrations. The uptake of the two hexosesis differentially inhibited by galactoac and by reagents thatcurtail ATP synthesis, and uptake of one hexose is not preventedby the other, even at a 100-fold excess. Sucrose uptake is muchslower than that of either hexose and is correlated with theappearance of invertase activity in the medium. Label from [¹⁴C-glc]-sucroseis taken up more rapidly than from [¹⁴C-fru]-sucrose. It isconcluded that these cells take up supplied sucrose only afterhydrolysis to hexoses, which are then absorbed by differentcarrier-mediated processes. Key words: Glucose, fructose, invertase, pea, protoplast, sucrose